This invention relates in general to pressure transducers (or transmitters) and more specifically to a transducer that is flush-mounted with a small fluid dead volume open to a fluid system whose pressure is to be measured.
In many industrial applications, such as in semi conductor, food and pharmaceutical products process control, a high degree of cleanliness is required. The ability to clean a system is correlated, in part, with a minimum of dead volume of fluid associated with the pressure measurement. A dead volume is typically a region of comparatively stagnant fluid in a transducer between the fluid held in a container or conduit and a pressure responsive element, typically an edge mounted diaphragm. Dead spaces affect the cleanliness of the system because any dead spaces in the conduit can trap a portion of the fluid and collect contaminants. When measuring the pressure of a fluid flowing in a conduit, dead volumes may also interfere with the smooth, laminar flow of the fluid. Cleanliness is a particularly important consideration in semi-conductor manufacturing processes where it is important to know the fluid pressure, but it is also necessary to maintain extreme purity of the fluids even though different fluids may flow through the same conduit at different times, e.g. up to one part per billion. In addition to control of dead volumes, it is also important to have a minimum surface area of the transducer or transmitter exposed to the fluid, and any exposed surface should have a finish that is highly smooth.
To provide this extreme level of purity it has heretofore been necessary to dismantle the conduit and/or pressure transducer manually, and then clean all of the regions accessible to the fluids. This cleaning operation is costly and time consuming since the semi conductor manufacture is stopped during the cleaning. There are also many situations, regardless of cleanliness requirements, where a fast dynamic response is essential. This operational characteristic requires a minimum volume of fluid in the transducer. (As used herein, transducer will be understood to include devices known in the trade as transducers which produce a voltage signal output in response to an applied fluid pressure input and devices known in the trade as transmitters which produce a current signal output.)
A pressure sensor in such process control applications is typically mounted in a threaded opening formed in the wall of the container or conduit, or onto a fitting mounted on the outside surface of the wall. In either case, conventional sensors have a closed end and there is a significant dead space within the sensor where fluids are trapped and non-fluid contaminants can collect. Also such sensors have a comparatively large surface area exposed to the fluid which can trap contaminants.
It is also advantageous for a pressure sensor to be of small size. Typically, the smaller the size, the easier it is for the transducer to fit into a constricted operating environment and attach to a wide variety of sizes and shapes of conduits and containers.
One well known device for measuring closed end fluid pressure is the Bourdon tube. The tube is a closed end conduit which is curved. Its open end is connected to the fluid whose pressure is to be measured. The application of a fluid pressure to the interior of the tube produces a hydraulic or pneumatic force which tends to straighten the tube. The degree of movement of the tip of the tube measures the applied fluid force. A limitation of the Bourdon tube is that because it has a closed end it cannot be used in flow through situations. It is inherently non compact and susceptible to thermal errors. Also it has a large surface area exposed to the fluid, a large volume and is difficult to clean.
The use of variable capacitance in pressure transducers to measure the applied pressure is also well known. One successful configuration is a "center mounted" pressure transducer described in U.S. Pat. No. 3,859,575 where one electrode is mounted to the center of a edge mounted diaphragm. This construction itself, however, does not solve the aforementioned, and often competing, design problems. If the transducer assumes the form shown in FIG. 1 of that patent, there is a substantial inactive volume of fluid within the transducer which leads to the cleanliness and dynamic response problems noted above. If the diaphragm is mounted adjacent the fluid in the conduit, as shown in FIG. 3 of that patent, the diaphragm is in contact with the fluid which raises the fluid compatibility problems noted above. Corrosion resistant materials in general are not the best in terms of the mechanical properties, particularly their elastic properties such as a good hysteresis response and low creep. Further, if the sensor is used in a high pressure range, the diaphragm exposed to the fluid must be relatively thick to withstand the fluid force. However, such a thick diaphragm will exhibit only a small deflection, which in turn will produce a small change in the measured capacitance with an attendant poor signal-to-noise ratio.
It is therefore a principal object of the present invention to provide a pressure transducer that is flush mountable to produce a very small dead volume of fluid within the transducer acting on a pressure responsive diaphragm and a comparatively small surface area exposed to the fluid.
Another object of this present invention is to provide a pressure transducer with the foregoing advantages that has a highly compact point of attachment to the conduit.
A further object of the present invention is to provide a diaphragm adjacent the fluid which is compatible with the fluid and yet which has an accurate and reliable response and which also can operate at both high and low pressure ranges.
Still another object of the present invention is to provide a transducer with a good dynamic response.
A further object of the present invention is to provide a transducer with the foregoing advantages that has a comparatively simple construction and a favorable cost of manufacture.